fix(core): add empty _configure to DockerContainer (#556)

I've used DbContainer in the past as a parent to a class where I
redefined `_configure()` and did not redefine `start()`. I'm now using
DockerContainer and wanted to follow the same pattern but it doesn't
have a `_configure()` function called in `start()`. I figure this gives
the easy option without being invasive?

tests pass

---------

Co-authored-by: David Ankin <daveankin@gmail.com>

core/testcontainers/core/container.py

@@ -87,6 +87,7 @@ def start(self) -> Self:
             Reaper.get_instance()
         logger.info("Pulling image %s", self.image)
         docker_client = self.get_docker_client()
+        self._configure()
         self._container = docker_client.run(
             self.image,
             command=self._command,
@@ -176,6 +177,10 @@ def exec(self, command) -> tuple[int, str]:
             raise ContainerStartException("Container should be started before executing a command")
         return self._container.exec_run(command)
 
+    def _configure(self) -> None:
+        # placeholder if subclasses want to define this and use the default start method
+        pass
+
 
 class Reaper:
     _instance: "Optional[Reaper]" = None